Uncertainties in mitigating aviation non-CO2 emissions for climate and air quality using hydrocarbon fuels†

The uncertainties over the effects of aviation non-CO₂ emissions on climate and air quality are assessed in the context of potential mitigation measures for liquid hydrocarbon fuels. Aviation non-CO₂ emissions that affect climate include nitrogen oxides (NO_x), aerosol particles (soot and sulphur-based), and water vapour. Water vapour and aerosols have small direct radiative effects but are also involved in the formation of contrails and contrail cirrus, currently, the largest non-CO₂ effect on climate. These non-CO₂ effects on climate are quantified with low confidence, compared to that of CO₂, which is quantified with high confidence. The sign of the NO_x radiative effects may change from positive to negative. The effects of soot and sulphur emissions on cloudiness are very poorly understood and studies indicate forcings that range from large negative through to small positive. NO_x and soot emissions can be reduced through changes in combustion technology but have tradeoffs with each other and CO₂. Soot can also be reduced through reduced aromatic content of fuels. In all cases, there are complex choices to be made because of tradeoffs between species, and CO₂. Contrail cirrus and soot aerosol–cloud interactions potentially have opposing signs but are both related to soot emissions (at present) and need to be considered together in mitigation strategies. Because of the uncertainties and tradeoffs involved, it is problematic to recommend definitive courses of action on aviation non-CO₂ emissions since they may be of limited effect or have unintended consequences. Aviation's non-CO₂ effects on climate are short-term, as opposed to those of CO₂, which last millennia. If aviation is to contribute towards restricting anthropogenic surface warming to 1.5 or 2 °C then reduction of emissions of CO₂ from fossil fuels remains the top priority. In terms of air quality, the situation is more straightforward with emissions standards being set by the International Civil Aviation Organization for NO_x and non-volatile particulate matter (and other minor species), which need to be complied with.